Considerable differences between predicted and actual – measured in the field – deformation of subsoil create premises for hypothesis that compressibility characteristics of overconsolidated cohesive soils obtained from oedometer tests are incapable to reflect accurately soil behaviour under loading in the field. It results from the fact, that oedometer incremental loading test procedure is simplified to such extent that it does not account for many factors contributing to actual compressibility characteristics. The paper describes an approach to quantification of degree of saturation influence on compressibility characteristics. Test results of overconsolidated medium plasticity clays sampled from route of Mińsk Mazowiecki ring road. It was shown that standard procedure for determination of compressibility parameters does not account for change in degree of saturation during loading, what results in errors in prediction of soil structure interaction. Some recommendation concerning how to account for change in degree of saturation during oedometer test were given.

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